1. Field of the Invention
The present invention relates to an image forming apparatus such as copying apparatus, laser beam printer, or the like.
2. Description of the Related Art
Hitherto, as a heating device (fixing apparatus) which is mounted in a copying apparatus or a laser beam printer, a film heating type apparatus disclosed in Japanese Patent Application Laid-Open No. S63-313182, H04-044075, or the like has been proposed and put into practical use.
According to such a film heating type fixing apparatus, a heat resistant thin film (fixing film) as a rotor for heating is closely adhered to a heating member by a rotor for pressing (pressing roller) and slid and conveyed, and a pressure-contact nip portion is formed by the heating member and the pressing roller so as to sandwich the fixing film. A material to be heated (hereinafter, referred to as a material to be heated) (transfer paper/sheet) which holds a non-fixed image is introduced to the nip portion, the material to be heated is conveyed together with the fixing film, and the non-fixed image is fixed as a permanent image onto the material to be heated by heat which is applied from the heating member through the fixing film and by a pressing force of the nip portion.
In the film heating type fixing apparatus, since the whole fixing apparatus can be constructed by a material of a low calorific capacity, electric power saving and a reduction in waiting time (quick start performance) can be realized.
For example, a plate-like ceramic base material of a low calorific capacity such as alumina (Al2O3), aluminum nitride (AlN), or the like is used for a base of the heating member. A heat generating pattern part using silver/palladium (Ag/Pd), Ta2N, or the like and a power feeding electrode pattern part made of a material of a low resistance such as Ag or the like for electrifying the heat generating pattern part are formed onto one surface of the base material by screen printing or the like. Further, the surface formed with the heat generation circuit part is covered by a thin glass protecting layer.
The heating member generates the heat by electrifying the heat generating pattern part from the power feeding electrode pattern part and a temperature of the whole heating member rises rapidly. The risen temperature of the heating member is detected by a thermistor arranged in contact with or arranged near the heating member and is returned to an electrification drive control part. The electrification drive control part controls the electrification of the heat generating pattern part, thereby maintaining the heating member temperature detected by the thermistor to an almost predetermined temperature (target fixing temperature). That is, the electrification drive control part heats the heating member and controls its temperature so as to reach a predetermined fixing temperature.
Since the calorific capacity of such a kind of fixing apparatus is low, it is excellent in a rapid start. On the other hand, since the calorific capacity is low, the fixing apparatus has such a drawback that a heat amount which is taken away from the heating member differs largely in dependence on the portion. That is, in the case where a width in the longitudinal direction of the material to be heated (target which is heated by the heating member) (length of material to be heated in the direction which perpendicularly crosses the conveying direction of the material to be heated) is relatively narrower than a length of the heating member in the longitudinal direction, when comparing an area where the material to be heated passes (sheet pass-through part) with an area where the material to be heated does not pass (non-sheet pass-through part), the heat amount which is taken away from the heating member in the former area and that in the latter area differ remarkably. Although the heat amount is taken away by the material to be heated in the sheet pass-through part, it is not taken away by the material to be heated in the non-sheet pass-through part. As an example of the case where the width of material to be heated in the longitudinal direction is relatively narrower than the length of the heating member in the longitudinal direction, a case where the maximum sheet passage width is equal to a width of lateral pass-through sheet of the A4 size [width in the longitudinal direction is equal to 297 mm] and the width of material to be heated in the longitudinal direction is equal to a LEGAL size [width in the longitudinal direction is equal to 215.9 mm], a longitudinal pass-through sheet of the A4 size [width in the longitudinal direction is equal to 210 mm], or the like corresponds to such an example. In the present invention, the material to be heated whose width in the longitudinal direction is relatively narrower than the length of the heating member in the longitudinal direction is referred to as a material to be heated of a “small size” or “narrow width size” hereinbelow. Therefore, when the material to be heated of the narrow width size is allowed to pass through, the temperature of the non-sheet pass-through part where the heat amount is not taken away by the material to be heated rises gradually and what is called a phenomenon of temperature rising at the non-sheet pass-through part occurs. In the film heating system of the low calorific capacity, the temperature rising is large and exerts an adverse influence on the quality.
As one of the quality problems which are caused by the temperature rising at the non-sheet pass-through part, material to be heated wrinkles (also simply referred to as “wrinkles” or “paper wrinkles” hereinbelow) at the time of a continuous sheet passage of the material to be heated of the narrow width size can be mentioned. In the film heating system, since the temperature difference is caused between the sheet pass-through part and the non-sheet pass-through part due to the temperature rising at the non-sheet pass-through part, as for an outer diameter of the pressing roller which is generally made of an elastic member that is thermally expanded, a large difference is liable to occur between the sheet pass-through part and the non-sheet pass-through part. Therefore, a conveyance variation in the longitudinal direction of the film and, consequently, a conveyance variation of the material to be heated occur and the wrinkles are liable to occur in the material to be heated. It has been known that the wrinkles are particularly liable to occur in an environment such as a high-temperature and high-moisture environment or the like in which a rigidity of the material to be heated such as paper or the like is weakened.